Ceramic packing element

ABSTRACT

The invention provides an improved ceramic packing element having the basic shape of a cylinder with an aspect ratio, defined by the diameter to length dimensions that is from 2.7 to 4.5.

BACKGROUND OF THE INVENTION

This invention relates to packing elements of the type that are oftencalled “random” or “dumped” packings. Such packings are used to filltowers units in which mass or heat transfer processes occur. Aparticularly important application is the use of such ceramic elementsin heat recovery operations where it is necessary to provide maximumeffective contact with hot fluids passing through the reactor. Anotherkey factor in maximizing efficiency is the maintenance of as low apressure difference between top and bottom of the tower as possible. Toensure this the packing elements should present the minimum resistanceto flow. This is promoted by very open structures but open structurealone is of limited use if the elements in the tower nest together suchthat parts of one packing element penetrate within the space of a secondelement. It is therefore important that the design of the elementsminimize the tendency of the elements to nest together.

The present invention relates particularly to ceramic packing elementsthat are produced by an extrusion or a dry-pressing process and hencehave an essentially uniform cross-section along one axial directionwhich provides an axis of symmetry for the element. Several such shapeshave been described in the art ranging from the very simple to thecomplex. All are based on an essentially cylindrical shape and differbasically in the internal structure within the cylindrical shape. Thesimplest structure is a basic cylinder with no internal structure atall. This type of structure is often called a Raschig ring and has beenknown for many years. At the other end of the complexity scale are thestructures described in U.S. Design Pat. No. 455,029 and U.S. Pat. No.6,007,915. Between the extremes there are simple wagon-wheel shapes suchas are described in U.S. Pat. Nos. 3,907,710 and 4,510,263.

It has now discovered that the dimensions of these shapes are criticalto achieving optimum performance and the range of dimensions for suchoptimum performance has not been taught in the prior art. U.S. Pat. No.4,510,263 teaches L:D ratios of 0.5 to 5 but indicates that preferredratios are from 0.5 to 2. In U.S. Pat. No. 3,907,710 there is noteaching on the aspect ratio but the Examples use elements with theratio of 1. U.S. Pat. No. 4,337,178 which teaches cylindrical ceramicelements as catalyst supports, also does not teach specific preferredaspect ratios but utilizes in the Examples elements with aspect ratiosof 2.11.

DESCRIPTION OF THE INVENTION

The present invention provides a ceramic packing element having anessentially cylindrical shape with an axis of symmetry in the directionof extrusion defining the length of the element and a greatest dimensionperpendicular to the length defining the diameter of the element inwhich the ratio of the diameter to the length is from 2.7 to 4.5 andpreferably from 3.0 to 4.4.

The element is defined as having a containing structure that isessentially cylindrical shape and this is understood to include shapesin which a round cylindrical shape has been somewhat flattened to createan oval cross-section as well as polygonal shapes with at least fivesides. The space within the containing structure can have a plurality ofsepta or none but since a primary application is in the field of heattransfer in which surface area becomes very significant, it is preferredthat significant internal structures are provided. In the context ofthis invention the term “septum” (plural “septa”) is used to describestructural member connecting one interior part of the cylindricalcontaining-structure with another. It therefore includes structures withlengths up to and including diameters.

The ceramic elements of the invention can be formed from any suitableceramic material such natural or synthetic clays, zeolites, cordierites,aluminas, zirconia, silica or mixtures of these. The formulation can bemixed with bonding agents, extrusion aids, pore formers, lubricants andthe like to assist in the extrusion process and/or to generate thedesired porosity or surface area for the intended application.

The elements can be used in mass transfer applications or as bases uponwhich catalytic components are deposited. The elements are particularlysuitable for mass transfer applications involving heat recovery fromstreams of hot gases. An example of such an application is found inthermal regenerators attached to plants whose function is to burn offany combustible material from a waste gas stream. In such regeneratorsit is vital for efficient operation that the heat values from theexhaust gas stream be used to heat up the incoming waste gas to betreated so as to minimize the cost of fuel required to burn off thecombustible material. The present invention teaches a way to optimizethe element design to achieve this end.

The elements can however be used with advantage in any application inwhich the surface area is an important factor in determining theefficiency with which the elements perform their assigned task.

DRAWINGS

FIG. 1 is a perspective view of a packing element of the invention froma point below and to one side, looking directly at one of the corners.

FIG. 2 shows a top view of the same element as is shown in FIG. 1.

FIG. 3 is a graph of percentage thermal recovery against the aspectratio of the elements

FIG. 4 is a graph of the percentage thermal recovery against gas flowrate.

DESCRIPTION OF PREFERRED EMBODIMENT

The invention is now more particularly described with reference to theembodiment illustrated in the Drawings. This is not intended to implyany necessary limitations in the scope of the invention because it willbe readily appreciated that many minor variations could be made withoutdeparting from the essential spirit of the invention.

A particularly preferred structure according to the invention comprisesa containing structure that is hexagonal with each pair of opposedcorners connected by a septum and parallel septa on either side connectthe sides meeting at those opposed corners. The overall effect is toprovide a plurality of triangular passages through the element, each ofessentially the same dimensions. An element of this design isillustrated in the Drawings.

The Drawings show a packing element, 1, with a hexagonal containingstructure, 2. A plurality of septa, 3, divide the interior space into aplurality of identical triangular passages, 4, through the element. Theelement has a length along the axis of symmetry, L, and a greatestdimension perpendicular to the axis of symmetry, D. In the Drawings theratio of D:L is about 4.

To demonstrate the significance of the D:L ratio, ceramic elements wereprepared with the same D but with three different values of L. Theseelements were then placed in a tube through which a gas stream at 1500°F., (815.6° C.), was passed at a rate of 207 ft/sec., (74.8 m/sec), andthe percentage of the thermal energy in the stream recovered through thecontact with the media was measured and plotted against the aspectratio. The results are shown in FIG. 3. The percentage thermal energyrecovery was then determined as a function of the gas velocity for eachof the three elements. The results are shown in FIG. 4. From thesegraphs it is very clear that there is a peak in percentage thermalenergy recovery between about 2.7 and 4.5 aspect ratios and particularlybetween about 3.0 and 4.4 and that this advantage is maintained at allthe gas flow rates tested.

This result is completely unexpected since there is no prior artteaching that points in this direction. It is also clearly extremelyadvantageous that as much thermal energy as possible be recovered byeach passage though the elements.

What is claimed is:
 1. A ceramic packing element having an essentiallycylindrical structure with a polygonal cross-section having at least 5sides, with an axis of symmetry in a direction defining the length ofthe element and a greatest dimension perpendicular to the lengthdefining the diameter of the element in which the ratio of the diameterto the length is from 2.7 to 4.5, the cylindrical structure enclosing aplurality of septa to define a plurality of passages through theelement, each of the passages having essentially the same length.
 2. Aceramic packing element having an essentially cylindrical structure withan axis of symmetry in a direction defining the length of the elementand a greatest dimension perpendicular to the length defining thediameter of the element, the ratio of the diameter to the length beingfrom 3.0 to 4.4, the cylindrical structure enclosing a plurality ofsepta defining a plurality of passages through the element, each of thesepta in the packing element being connected at first and second endsthereof with the essentially cylindrical structure and intersects aplurality of other septa at spaced locations along its length.
 3. Aceramic packing element according to claim 2 in which the plurality ofpassages through the element each have essentially the same length.
 4. Aceramic packing element according to claim 2 in which the cylindricalstructure has a polygonal cross-section having at least 5 sides.
 5. Aceramic packing element according to claim 4 having six sides in whichthe element is provided with a plurality of internal septa definingidentical passages through the element each of which has a triangularcross-section.
 6. A ceramic packing element according to claim 1 inwhich the ceramic is made from materials selected from the groupconsisting of natural or synthetic clays, aluminas, zeolites,cordierite, zirconia, silica and mixtures thereof.
 7. A method ofrecovering thermal energy from a gas stream, the method comprisingpassing a heated gas stream through a plurality of ceramic packingelements, each of the ceramic elements including an essentiallycylindrical structure having a polygonal cross-section having at least 5sides, with an axis of symmetry in a direction defining the length ofthe element and a greatest dimension perpendicular to the lengthdefining the diameter of the element in which the ratio of the diameterto the length is from 2.7 to 4.5, the cylindrical structure enclosing aplurality of septa to define a plurality of passages through theelement, each of the passages having essentially the same length, theceramic packing elements recovering heat from the gas stream.